Level detectors are used to measure the level of a liquid in a vessel and, in function, the detectors are usually locating a boundary between the liquid in the vessel and the gas above it. Thus, level detectors may also be described as boundary detectors where the upper surface of the liquid is the boundary that is detected.
In some level detectors, such as disclosed in U.S. Pat. Nos. 4,418,035; 4,406,011; and 4,440,717, sensors are placed at various levels within a vessel and they determine the presence or the absence of the liquid at a particular location. Basically, these devices determine whether a sensing element is disposed in the liquid or in the gas above the liquid. The sensing element is usually heated at a constant rate and the temperature of the sensing element is monitored. Since the liquid will absorb heat from the sensing element faster than air or gas, the temperature of the heated sensing element will be indicative of the presence or absence of the liquid. Thus, by measuring the temperature of a heated sensing element, these devices determine the presence or absence of the liquid at a particular location.
One of the disadvantages of level detectors such as disclosed in the aforementioned patents is the size and expense of the detector. For example, in U.S. Pat. Nos. 4,418,035 and 4,406,011 a thermocouple circuit containing two wires is required for each position to be monitored. Since it is usually desirable to monitor a number of positions in order to accurately determine the level of a liquid, these devices require numerous wires. Because of the number of needed wires, the level detector must have a relatively large cross-section or the wires must be made very small. Of course, small wires are both expensive to manufacture and delicate to use.
In one embodiment of the present invention, an analog level detector or boundary detector provides an accurate indication of the level of the liquid within a vessel without the attendant size and expense problems associated with known level detectors of this type. The apparatus is designed for detecting the location of a boundary between two substances having different thermal absorption properties. An elongate sensor is disposed across the boundary between the two substances and the sensor is constructed of a plurality of a-type segments and b-type segments. These segments are connected with alternating types being connected end-to-end so that a series of a-type to b-type junctions are formed along the sensor. The a-type segments are constructed of a material different from the b-type segments so that thermocouple junctions are formed therebetween. A segmented heater cable is provided for heating selected ones of the thermocouple junctions. Preferably, the heater cable heats every other one of the thermocouple junctions so that a series of thermocouple pairs (a heated junction and a cold junction) are created such that the total voltage potential across the sensor is the sum of the differential voltages created by the thermocouple pairs. Electrical insulation is provided to insulate the sensor from the heater cable and to insulate the heater cable and the sensor from the two substances in which the sensor is disposed. The electrical insulation is thermally conductive so that heat is transferred from the heater cable to the sensor and from the sensor and heater cable to the substances in which the sensor is disposed.
When a heated thermocouple junction is disposed in a substance having a low thermal absorption property, such as a gas, the heated junction will have a relatively high temperature compared to the cold junction and a voltage potential will appear across the thermocouple pair. When the heated thermocouple is disposed in a substance having a relatively high thermal absorption characteristic, the heated thermocouple will be cooled to a relatively low temperature. In such case, the thermocouple pair will have a greatly reduced voltage potential. Since the voltage potential across the sensor will be the sum of the voltage potentials produced by the thermocouple pairs, the voltage potential across the sensor will vary inversely with the number of heated thermocouple junctions that are disposed in a substance having a high thermal absorption characteristic. In the case of a sensor disposed to detect the level of a liquid, the voltage potential across the sensor will grow smaller as the liquid level rises because the liquid will cool the heated thermocouple junctions. Thus, to measure the location of a boundary or the level of a liquid, a meter is provided to monitor the voltage across the sensor. In this embodiment, multiple positions or levels may be monitored by a single sensor wire and one return wire. Thus the present invention results in significant savings in terms of size and expense when compared to known conventional thermocouple type detectors.
In accordance with another aspect of the invention, a digital detector includes a plurality of measuring wires disposed in a parallel relationship with each of said measuring wires being constructed of a-type segments and b-type segments. These segments are connected with alternating types connected end-to-end so that a series of a-type to b-type junctions are formed in said plurality of measuring wires. As before, these junctions are thermocouple junctions.
A segmented heater cable is used to heat each of the thermocouple junctions in the plurality of measuring wires. Electrical insulation is provided for insulating the measuring wires, and the insulation is thermally conductive for transferring heat from the heater cable to the measuring wires and from the measuring wires and heater cable to the environment in which the measuring wires are disposed. Thus, as before, the temperature across the heated junctions is dependent upon the substance in which the junctions are disposed. The thermocouple junctions in the plurality of measuring wires are disposed so that the voltage potential across the plurality of measuring wires is a digital representation of the position of the boundary between two substances or of a level a coolant in a vessel. A circuit is provided for detecting the voltage potential across each of the plurality of measuring wires and for producing a digital representation of the position of the boundary.
In the digital embodiment, the minimum number of measurement wires needed to monitor x locations is 2.sup.X plus a return wire. For example, two measurement wires are needed to measure four positions and three measurement wires are needed to measure eight positions. Thus, the digital embodiment also results in a substantial savings of size and expense relative to known detectors.
In accordance with yet another aspect of the invention, the detector may be constructed having a plurality of measurement wires disposed in a vessel that sense the presence and absence of a coolant only at one level. In such embodiment, the sensor wires are constructed primarily of a-type material with one segment of b-type material which forms one upper and one lower thermocouple junction in the measurement wire. A segmented heater cable is used to heat the upper thermocouple junction and the lower thermocouple junctions remain cool. When the coolant level is below an upper thermocouple, a voltage potential appears across the measurement wire, but when the coolant reaches the upper thermocouple, both the upper and lower thermocouple are essentially the same temperature and the voltage potential across the measurement wire is about zero. In this embodiment, a common return wire is shared by all measurement wires and the voltage potential appearing on the measurement wires relative to the common wire will indicate the coolant level. In this embodiment, only one wire is needed for each monitoring position, and all wires share a common return. Thus, this embodiment also results in a savings as to the number of needed wires and attendant expense as compared to known devices.
The above described analog, digital and discrete embodiments may also be combined into one system. In such case, each of the three systems would share a common return wire and a common segmented heater cable. In such embodiment, the devices could be cross checked as a quality control measure.